The present invention relates to a cell-voltage measuring structure for a fuel cell stack and its related method and, more particularly, to a cell-voltage measuring structure for a fuel cell stack provided with a plurality of stacks of unit cells including conductive separators and its related method. A fuel cell system, which serves as a device that directly converts chemical energy of fuel into electrical energy, includes a pair of electrodes, between which an electrolyte membrane is sandwiched, an anode electrode of which is supplied with fuel gas, containing hydrogen, while the other cathode electrode is supplied with oxidizer gas, containing oxygen, to cause electrochemical reaction, described below, to occur on surfaces of the pair of these electrodes on respective sides facing the electrolyte membrane for extracting electrical energy from the electrodes (see Japanese Patent Application Laid-Open Publication No. 8-106914).Anode Reaction: H2→2H++2e−  (1)Cathode Reaction: 2H++2e−+(½)O2→H2O  (2)
It is known that fuel gas is supplied to the anode electrode by a method in which hydrogen is directly supplied from a hydrogen storage tank or a method in which hydrogen-containing fuel is reformed to supply hydrogen-containing gas. Examples of the hydrogen storage tank include a high-pressure gas tank, a liquid hydrogen tank and a hydrogen-absorbinig alloy tank. Examples of hydrogen-containing gas include natural gas, methanol and gasoline. On the other hands as for fuel gas supplied to the cathode electrode, air is commonly utilized.
With such a fuel cell system, the pair of electrodes are sandwiched between conductive separators to form a unit cell and a plurality of unit cells are stacked into a fuel cell stack that is normally used. Here, a need arises for monitoring (measuring) a cell voltage of each unit cell with a view to confirming whether normal electric power generation takes place and controlling the flow rates of reaction gases based on the cell voltages upon which in the presence of abnormal voltages, a fail-safe function is rendered operative in the system.
Japanese Patent Application Laid-Open Publication No. 9-283166 proposes a cell-voltage measuring structure wherein side faces of the separators of each unit cell are formed with two round holes, that is, an anode separator and a cathode separator of the unit cell are formed with one round hole one-by-one, to which monitor pin terminals are inserted for monitoring the cell voltage.
Japanese Patent Application Laid-Open Publication No. 2002-358993 proposes a structure wherein a metallic plate, formed in a C-shape with a spring property, is clamped onto concaved portions formed on separators to monitor a voltage of each unit cell.
Japanese Patent Application Laid-Open Publication No. 2003-86205 proposes a cell-voltage measuring structure wherein a resiliently deformable cell-voltage monitoring member is depressed against a dent formed in a unit cell to cause resilient deformation whereby a contact between the unit cell and the cell voltage monitoring member is prevented from loosening.